1. Field of the Prior Art
The invention relates to shared lines equipment (Vorfeldeinrichtung), especially for a B-ISDN switching system having subscriber stations, wideband concentrators, and a central wideband switching network.
2. Prior Art
A large-scale time-division switching arrangement often consists of a central switching network and a series of so-called concentrators, which are connected to the in/outputs of the central switching network. The concentrator is a shared lines equipment (Vorfeldeinrichting), effecting a concentration of the traffic offered. For the subscriber stations connected to the concentrator there is nonzero loss on average. The number of junction lines between the concentrator and the switching arrangement can be determined such, that the handling of the traffic offered by the subscriber stations connected to the concentrator is affected only insignificantly. In practice, this means that the number of the subscriber stations to be connected to the concentrator with a predetermined loss must not exceed a certain limit.
For the subscriber stations connected through the concentrator it is disadvantageous that a wiring of the concentrator with lines having different traffic volumes has a negative effect on the operating conditions. Therefore, a system structure is often conceived, in which a division of the switching arrangement is made, especially in a B-ISDN switching system, into a narrow band and a wideband portion. Both portions consist of the central switching network with inserted concentrators. The switching arrangement comprises a common control arrangement, which supplies adjusting and control information to the concentrators through an associated interface circuit.
From Nachrichtentechnik-Elektronik, 1985, Vol. 2, pp. 43 and 44 a digital switching exchange of a modular configuration is known having a distributed control for the through-switching of narrow band channels. For different applications the digital switching exchange has various types of modules, for example, transit modules and concentrator modules. These shared lines equipments communicate with each other over a central bus operating in the time-division mode, the communication data being transmittd between the modules according to the principle of the line storage and the process data in the framework of speech communication being transmitted in the respective time slots in the parallel mode over the process bus.
The concentrator module has a concentrator stage, a microcomputer control, an interface unit, a time-directional coupler and a bus interface.
The transmission in such a system configuration via a B-ISDN switching system is very expensive in the concentrators, especially because the concentrators have their individual microcomputer controls which have to communicate with each other for switching the connections through over the bus system operating in the time-division mode.
Consequently, as already explained hereinbefore for an H1 switching, a central wideband switching network for 1024 2 Mbit/s channels, for example, can be provided to which up to 16 wideband concentrators can be connected.
The wideband concentrators do not affect an internal switching, that is to say, without the help of the central wideband switching network no switching takes place between the connected groups of subscribers. All data reaching a wideband concentrator through a highway H1 (H2), leave it again through a highway H2 (H1).
A highway H is a bidirectional multiplex connection via which N channels are conveyed each time. For example, the transmission rate of the connection line between the central wideband switching network and the wideband concentrator is 139.264 Mbit/s with a word clock of 17.408 MHz. This is equal to a multiplex frame of 68 time slots of 2.048 Mbit/s each, whereas in the above case only 64 time slots are switchable. The remaining four time slots are reached through the special-purpose control interface circuit. The multiplex frame on the line of the subscriber group has, for example, 34 time slots. Four H1 channels from these time slots are allocated to each subscriber for wideband and 1/8 H1 channel (equal to 256 kbit/s) for narrow band, whilst the clock synchronization of the two systems is effected via one common clock, whereas, on the other hand, the switching of narrow band and wideband data channels is effected entirely separately.
In the system configuration depicted hereinbefore, each concentrator contains 16 concentrator time-stage modules, which are each time allocated to a group of 8 subscribers. The concentration factor is then 8. According to the (4,2) concept known from the DE-OS 35 37 451, four data bits are transmitted in parallel and bidirectional per time slot.
Each concentrator time-stage module has four bidirectional time stages for switching 34 2 Mbit/s channels, enabling in the system configuration described above the switching of 8* (4H1) channels of one group (as well as two 2 Mbit/s channels bundling narrow band channels) onto the 68 channels, which are available in a time-division multiplex frame.